The reactor period, T, is the time required for the power level of a nuclear reactor to increase by a factor e (the base of natural logarithms). When the reactor period becomes too short, i.e., the power level is increasing too rapidly, a reactor scram should be initiated. Existing rate of change of power level monitoring circuits measure the reactor period by using an appropriate circuit to obtain the logarithm of the counting rate and subsequently differentiating this signal with respect to time. If the reactor is on an asymptotic period, the resulting signal is a measure of the reactor period. At low counting rates, the statistical fluctuations associated with the conventional log count rate and differentiating circuits are large. Either excessively long time constants must be incorporated in the log count rate and differentiating circuits or relatively frequent spurious scram signals must be accepted. Further, conventional circuits have been required to utilize primarily analog rather than digital circuit elements.
At low and medium reactor power levels, it is acceptable to allow modest increases in power level before initiating protective action even though the reactor power level may be increasing too rapidly, provided the protective action is very fast after the acceptable power level increase has been reached.
It is, therefore, an object of this invention to provide a rate of change of reactor power level monitoring circuit by means of a count-factor-increase time monitoring circuit. The rate of change of reactor power level as measured by this circuit can be related to an asymptotic reactor period.
Another object of this invention is to provide a digital rate of change of reactor power level monitoring circuit operable at low power levels with reduced numbers of spurious reactor scram signals.
A method and means is described for monitoring the rate of change of reactor power level. The output of a pulse-type neutron detector is utilized by comparing the number of neutron-induced events which occur within any preselected, fixed time interval to the number of neutron-induced events which occurred during the immediately-preceding equal length time interval. If the second count is larger than the preceding count by any preselected factor greater than unity, then the reactor power level is deemed to be increasing at an unacceptable rate and a reactor scram is to be induced.